1. Field of the Invention
This invention relates to a photometric control device for a camera in which a photometry is conducted on a subject field which is divided into plural photometric regions, thereby calculating an exposure value on the basis of information on the brightness of each photometric region.
2. Description of Related Art
There has been conventionally known a camera for performing photometry on a subject field to be photographed which is divided into plural photometric regions to calculate an exposure value on the basis of information on brightness of each photometric region. For example, a camera as disclosed in Japanese Laid-open Patent Application No. 1-217428 adopts a so-called "divisional photometry" in which a photographic field (corresponding to a subject field) is sectioned in a matrix form into plural photometric regions, and the photometric region is independently subjected to a photometry to detect information on the brightness of each photometric region, that is, a photometric signal (photometric output value) corresponding to each photometric region and obtain a proper exposure value.
As a photometric control device for the divisional photometry it has been known to use an electric charge accumulation-type of photoelectric conversion device, such as a Charge Coupled Device (CCD) as disclosed in Japanese Laid-open Patent Application No. 62-259022, for example.
This photometric control device using the above kind of accumulation-type of photoelectric conversion device has the following disadvantage when it is used to carry out a photometry under a fluorescent lamp.
Under an artificial illumination such as a fluorescent lamp, illumination light is periodically fluctuated as shown in FIG. 7 due to affection of the frequency of a power source, that is, a flicker phenomenon occurs. The flicker period Tf is about 10 ms in a region where a 50 Hz power source is used, and about 8.3 ms in a region where a 60 Hz power source is used. In a case where a photometry is carried out under illumination having such a flicker phenomenon, the photometric output would be fluctuated in accordance with a phase of the flicker period at which the electric charge accumulation is conducted if the electric charge accumulation time t of is shorter than the flicker period Tf, so that no accurate brightness may be obtainable.
Therefore, in the photometric control device as disclosed in Japanese Laid-open Patent application No. 62-259022, the electric charge accumulation operation is carried out plural times, and the respective results of electric charge accumulation operation are averaged to calculate the brightness value, whereby the affection of the flicker is minimized.
The flicker greatly affects the photometric result in the case where the electric charge accumulation time is within a predetermined range, and in other cases, an accurate brightness value can be obtained in with no consideration of the affection of the flicker. However, in the photometric control device as disclosed in the above publication, the average value of the electric charge accumulation time is calculated to obtain the brightness value at all times irrespective of the length of the electric charge accumulation time. Therefore, an unnecessary operation must be carried out even when no affection of the flicker is imposed, and thus the brightness value can not be rapidly obtained.